There is a pressing need to understand how child sexual abuse (CSA) affects adolescent females' sexual risk behavior. Adolescent females have disproportionately high rates of sexually transmitted infections, largely due to heterosexual contact (CDC, 2008). CSA females are especially vulnerable, as they tend to initiate sex at early ages, engage in higher rates of sexual risk behavior, and benefit less from traditional risk reduction efforts than their non-abused peers (Senn et al., 2008; Greenberg, 2001). Although the sexual risks associated with CSA begin in adolescence, little is known about the early sexual development of CSA youth or the mechanisms by which CSA confers heightened risk. This K01 Career Development Award proposes a plan of training and mentored research experiences that will lay the foundation for an independent program of developmentally-informed prospective research directed at articulating mechanisms of sexual risk among CSA youth. Training will be acquired in (1) sexual risk and risk-reduction; (2) methods and issues in conducting prospective longitudinal research with CSA youth; (3) physiological assessment of affect regulation; and (4) advanced longitudinal data analytic techniques. Training will occur in a resource-rich environment that encourages innovative and collaborative research. Training is also integrated into the research plan, which includes two studies that seek to identify CSA-specific pathways to emergent sexual risk. Study 1 examines longitudinal associations between psychopathology (PTSD and externalizing problems), physiological regulation of trauma-related affect, and sexual risk behavior using extant data from a long-term prospective study of CSA. Study 2 entails a new prospective data collection to examine trajectories of sexual risk during early adolescence and test two potential risk mechanisms: (1) traumatic sexualization TS), the process by which CSA distorts cognitive and affective orientations towards sexuality, and (2) psychopathology (PTSD and externalizing problems. TS is conceptualized in terms of cognitive and affective distortions that are manifest in youths' subjective and physiological experiences. Participants will include 120 sexually abused and non- abused young adolescent females between the ages of 11-14 years who will be followed over an 18-month period. TS and PTSD are expected to play a unique role in the emergence of sexual risk among CSA youth. Externalizing behavior problems are expected to predict sexual risk among non-abused youth. The proposed work will increase our knowledge of the factors that increase sexual among CSA youth risk so that more effective risk reduction programs can be developed for this highly vulnerable population. Results will inform the development of an R01 proposal for a longer-term, more comprehensive study of the role that traumatic sexualization plays in sexual health within casual and romantic relationships.